Current sinks and regulators are used in a variety of applications and devices to provide a desired current to a circuit or device accurately and without large fluctuations. Some standards and devices require voltages and currents to be provided at accurate levels or within well-defined ranges, for operation and for detection and identification of characteristics or to determine whether a device is operating within a known standard.
One existing technology that makes use of such voltage and current regulation is Power over Ethernet for networked electronic devices. Electronic devices are often linked together via communication networks that allow the devices to communicate and share data. Local area networks (LANs), wide-area networks (WANs), the Internet, wireless networks, etc., are just some of the varieties of these communication networks. Several standard network implementations are widely used, such as Ethernet for copper cables used in LANs connecting multiple computers or other devices.
It is convenient for some devices to receive power over a network communication link, such as an Ethernet cable, rather than obtaining power from a separate source such as batteries or a standard electrical outlet. This allows the device to conveniently have a single cable connection for communication and operation. For example, Internet Protocol (IP) telephone devices can transmit voice data to another device over the Internet using IP or over other networks instead of over telephone wires. Like standard telephones, receiving power over the communication link allows the telephone to be provided in areas where there is no convenient separate power source. Other types of devices also benefit from receiving power over network links, such as wireless access points for wireless networking devices, video cameras, point-of-sale devices, security scanners or cameras, and other devices.
The Power over Ethernet (PoE) standard, also known as IEEE 802.3af, was created to meet the demand for devices that receive power over network communication links. PoE architecture consists of two elements: the power-sourcing equipment (PSE) and the powered device (PD). A powered device is a device that is drawing power or is requesting power over the data link. The PSE is the equipment that provides DC power to the powered device, and its main functions are to probe the network for powered devices, supply power to the device if a compatible powered device is detected, monitor the power on a link, and remove power from a link when power limits are exceeded or when a powered device is disconnected or no longer requests power.
The Power over Ethernet standard puts only minimal requirements on the powered device to acquire power from the power interface (PI). This helps to minimize the cost of these typically cost-sensitive devices so that the major burden of power control and device detection is put on the PSE, which is typically the more centralized, integrated and expensive part of the system. To prevent harm to non-compatible Ethernet devices by PoE technology, the 802.3af standard defines a powered device detection or “discovery” method. Power will only be applied by power-sourcing equipment if a powered device that is compatible with the standard has been successfully detected. The discovery process is effective at preventing power from being sent to devices inadvertently plugged into Ethernet outlets having PSE devices attached. The PSE can also classify a powered device before power is applied based on current levels in the powered device at predetermined voltage levels. A powered device classification or power class defines the maximum power required by the powered device during its operation and allows PSE devices to better manage limited power resources.
Since it is desired for the PSE to perform the detection and powering functions and for the powered device to minimize the cost of PoE components, the PoE components in the powered device should be simple and low-cost. However, one problem with currently-used PoE circuitry used in powered devices is that high-cost components are commonly used. For example, existing classification circuits, used in powered devices for enabling the PSE to classify the devices, use high cost operational amplifiers and high voltage level shift circuits that have been formed using “BCD” (bipolar-CMOS (Complementary Metal Oxide Semiconductor)-DMOS (Double Diffused MOS)) processes. For example, a high voltage, low dropout voltage regulator feeding a fixed resistor can be used for a classification circuit, but requires a bipolar “pass element” transistor, operation amplifier, voltage reference, and a level shifter including two or more transistor switches for level shifting the operational amplifier output to the pass element transistor's base-gate connection. These types of components significantly increase the cost of the powered device.
Accordingly, what is needed is an apparatus for providing an accurate current sink and current regulation in accordance with a desired standard, such as in a powered device for Power over Ethernet implementations, using simpler and more cost-effective components. The present invention addresses such a need.